1. Field of the Invention
The invention relates to an ink jet printing apparatus and printing method for printing characters or images on a print medium according to print data.
2. Description of the Related Art
Ink jet printing apparatuses are capable of handling high speed, high image quality printing because they are capable of ejecting relatively minute ink drops at a relatively high frequency. Ink jet printing apparatuses are excellent in respect to these points, in comparison to other printing methods. Among ink jet printing apparatuses, thermal ink jet printing type printing apparatuses, which eject ink using bubbles generated in the ink via a heater (an electro-thermal converter), have ejection ports (nozzles) formed on them at a high density. Thus, thermal ink jet type printing apparatuses are capable of performing high resolution, high image quality printing.
Among ink jet printing apparatuses, there are serial scan type ink jet printing apparatuses that form print images by repeating main scans of the print head and sub-scans of the print medium. Miniaturization of these serial scan type ink jet printing apparatuses is relatively easy, and for this reason manufacturing costs are low, and they are generally widely available.
In the case where printing is carried out by a thermal ink jet printing method, when high duty printing with a high printing frequency, etc., is performed, temperature within the print head becomes nonuniform in the direction in which the ejection ports are aligned, due to differences in the amount of heat dissipation at each of the ejection port locations, etc. When ejection of ink is performed by a thermal ink jet printing method (referred to below simply as an ink jet printing method), ink is ejected via the bubble pressure caused by generating bubbles within the ink. In doing so, the growth rate of the generated bubbles is influenced by the temperature of the ink present in the vicinity of the bubbles. When ink temperature is high there are many molecules from within the ink that become vapor and form bubbles, and the bubbles grow comparatively large. Because the bubbles at the time of ink ejection become large, the amount of ejected ink becomes large. Thus, when temperature becomes nonuniform with respect to the direction of the ejection port array, the amount of ink ejection concurrently varies with respect to the direction of the ejection port array, and because of this print image density becomes nonuniform.
With respect to the above, up until now, various proposals have been disclosed in order to attempt to suppress variation of the ejection amount, within an ejection port array. As a method of suppressing the occurrence of a temperature range, which causes ejection amount variation, a method is known whereby nonuniformity of the temperature distribution within an ejection port array is reduced via heating, by adding an amount of energy, such that ink is not ejected from the ejection ports, into heaters which is not used for ink ejection. In Japanese Patent Laid-Open No. 2008-168626, in order to further suppress variation in the amount of ejection, within an ejection port array, temperature at the ends of the ejection port array is detected, the temperature distribution within the ejection port array is estimated from the detected result, and the pulse width applied to the heaters is varied for each ejection port in accordance with the estimated temperature distribution. According to this method it is possible to suppress variation of the amount ejected within an ejection port array because, for each of the ejection ports of differing temperatures, it is possible to establish a driving pulse that is sufficient for a target ejection amount.
In the above method, however, because the detection of the temperature at both ends of the ejection port array is a prerequisite, a new structure becomes necessary in order to detect temperature. Also, in contrast to the structure of a general ink jet printing apparatus in which all of the ejection ports formed on the print head are heat driven with a pulse of the same width, a new structure is necessary in which differing pulse widths can be established for each of the heaters corresponding to the ejection ports, according to the detected temperature. When the above points are taken into account, apparatus complexity and cost increase are associated with performing printing according to the method disclosed in Japanese Patent Laid-Open No. 2008-168626.